These recent changes in the definition of Crepidula are based on both DNA sequence data as well as anatomical work. Dissections of various calyptraeids show that species that are now placed in Grandicrepidula and Maoricrypta are anatomically very different from the true Crepidula. If only the shells are examined this difference is not apparent. This distinction is supported by DNA sequence data from 3 genes (COI, 16S and 28S).[3]

The genus Crepidula is probably the best studied group within the calyptraeids. A variety of species are commonly used in developmental, ecological, and behavioral research.[4] They have been the major focus of research on protandrous sex-change in marine invertebrates and have been used to demonstrate that sex change is environmentally mediated (the timing of sex change depends on association with other individual snails).[5][6][7]Crepidula fornicata and Crepidula onyx are well-studied examples of invasive, exotic species in marine habitats.[8][9]

Due to their simple shells and plastic morphology, calyptraeid taxonomy is challenging. In many cases distinct species with similar-looking shells have been lumped into a single species with either global or unusual distributions. In these cases close examination of the mode of development or of DNA data is vital to verify the species identity. Because such taxonomic lumping is difficult to clear from the internet or from the literature many species range estimates available on the internet include dubious data or data from species that have been taken out of synonymy.

Species that were previously placed in the genus Crepidula but have subsequently been removed from the genus on the basis of work by Bruce Marshall of the Te Papa Museum and Rachel Collin of the Smithsonian Institution include the following:

Like all calyptraeids, slipper snails are sedentary filter-feeders. Adults use their large gill to capture microalgae from suspension, but there is some evidence that small juveniles can also use the radula to scrape algae from the substrate.

Crepidula fornicata - Native to the east coast of North America and ranging from Florida north into Canada. This species is now widespread and considered highly invasive along the north coast of Spain and France, along much of England's coastline and into the North Sea. It has also been reported from San Francisco Bay, Puget Sound and the Mediterranean Sea.

Crepidula convexa - Native to the east coast of North America and ranging from South Carolina to New England. This species has been reported from Puget Sound and San Francisco Bay.

Collin, R. 2002. Another Last Word on Crepidula Convexa With a Description of C. Ustulatulina N. Sp. (Gastropoda: Calyptraeidae) From the Gulf of Mexico and Southern Florida. Bulletin of Marine Science, 1: 177-184.